1. Field of the Invention
This invention relates to a cruise control system for a vehicle which automatically maintains a desired vehicle speed, and more particularly to such a cruise control system for an automatic transmission vehicle.
2. Description of the Prior Art
Recently, there has been put into practice a cruise control system which automatically keeps the vehicle speed at a desired speed set by the driver. In the control system, when the vehicle speed reaches a desired speed, the driver operates a setting means and thereafter, the throttle valve of the engine is automatically controlled according to the difference between the desired speed and the actual vehicle speed to maintain the desired speed.
Generally, the cruise control system is required to have four functions, a vehicle speed set function, an increase set function, a decrease set function, and a resume function. The vehicle speed set function is to fix the vehicle speed at the speed at the moment when the setting means, e.g., a vehicle speed set switch, is operated. The increase set function is to increase the set vehicle speed and is accomplished by, for instance, pushing an acceleration switch. The set vehicle speed is increased while the acceleration switch is kept depressed, and when the acceleration switch is released, the vehicle speed is fixed or set to what the speed was at the moment the acceleration switch was released. The vehicle speed set switch and the increase set switch are generally combined into a single switch so that when the single switch is momentarily pushed, the vehicle speed is fixed at what the speed was at the moment the switch was pushed, and when the switch is kept depressed, the vehicle speed is increased until the switch is released and the vehicle speed is fixed to what the speed was at the moment the switch was released. The decrease set function is to decrease the set vehicle speed and is accomplished by, for instance, pushing a coast switch. The set vehicle speed is gradually decreased while the coast switch is kept depressed, and when the coast switch is released, the vehicle speed is fixed or set to what the speed was at the moment the coast switch was released. The resume function is to resume, when the constant speed cruising is interrupted by a predetermined operation, e.g., operation of the brake pedal, the vehicle speed set before the interruption of the constant speed cruising. The resume function is accomplished by pushing a resume switch which causes the vehicle speed to revert to the speed set before the interruption of the constant speed cruising and to be fixed at the speed.
In U.S. Pat. No. 4,535,865, there is disclosed a cruise control system for a vehicle having an automatic transmission in which the transmission is downshifted by one speed to increase engine output torque when the vehicle is accelerated by the increase set function or the resume function, thereby improving acceleration during the increase set function or the resume function in the case of ascending a slope, for example.
However, the cruise control system of the United States patent is disadvantageous in that the acceleration effect due to the increase in the throttle opening by the increase set function or the resume function acts simultaneously with the acceleration effect due to the increase in the engine output torque by the downshifting, abruptly increasing acceleration force, thereby generating a shock effect at the beginning of the acceleration which adversely affects riding comfort.
On the other hand, in Japanese Unexamined patent publication No. 58(1983)-163022, there is disclosed a cruise control system for a vehicle having an automatic transmission in which the transmission is downshifted to increase the engine brake effect when the vehicle is decelerated by the decrease set function, thereby improving the deceleration effect during the decrease set function in the case of descending a slope, for example.
However, the cruise control system of the Japanese patent publication is disadvantageous in that the deceleration effect due to the reduction in the throttle opening by the decrease set function acts simultaneously with the deceleration effect of the downshifting to abruptly increase the deceleration force, thereby generating a shock effect which adversely affects the driving comfort.